The invention concerns a ring cavity laser device and, more precisely, a device using the optical beam emitted by a master laser to control or lock the operation of a slave laser located in a ring cavity.
This "injection locking" consists in controlling a laser hereinafter referred to as the "slave", generally powerful yet having poor coherence (longitudinal multimode) and divergence (transverse multimode), by a laser of lower power known as the "master", which is very coherent and whose divergence is limited by diffraction. To achieve this, the light from the master laser is injected into the cavity of the slave laser. Once the injection locking is established, the slave laser possesses the properties of coherence and divergence inherited from the master while conserving its power.
One of the difficulties of this technique is the necessary insulation between the slave and the master: the light from the master must be coupled to that of the slave very efficiently yet, on the other hand, the inverse must be avoided since the master would otherwise be disturbed. This is particularly true in the case of semiconductor lasers, where an insulation of more than 30 dB between slave and master is required.
To achieve this unidirectional insulation optical insulators using the Faraday effect are employed. These are large, heavy and expensive.
For example, devices in which a laser, called the slave, is constituted by an amplifying medium placed in a Fabry-Perot type optical cavity are well known. As shown in FIG. 1, the slave laser 3 with its cavity constituted by the mirrors 5 and 6, receives from a master laser 1 a master beam Fm. The wavelength of the slave laser and therefore that of the beam FS emitted by the optical cavity is controlled by the master beam Fm.
It is essential for the operation of the master laser 1 to take precautions against return of light from the slave optical cavity. This is achieved by use of an insulator 7.
Devices such as that shown in FIG. 2 are also known. These possess an optical ring cavity formed by the mirrors M6, M7 and M8 and containing the amplifying medium. The mirror M6 allows the injection of a master beam Fm, supplied by a master laser 1, into the ring cavity.
The mirror 8 is half-reflecting and allows the output beam FS to exit the cavity.
In addition, in injection locking devices, the frequency of the master laser and that of the slave, which both oscillate freely (without injection), must be approximately the same in order to fall in the locking range. To achieve this, we may choose between modifying the frequency of the master laser or that of the slave by adjusting, for example, the length of the ring.